Many implanted medical devices rely on electronic circuits to provide electric stimulation of nerves. Such devices include retinal implants and cochlear implants, where the electrical stimulation is used to transfer information to the brain, as well as devices for applications where the stimulation is used for motor control. For chronic use in patients, it is necessary that such devices transfer a zero net charge, and so the use of charge-balanced rectangular biphasic current pulses for neural stimulation is well established. These pulses often comprise a constant current stimulating cathodic phase, followed by a brief interphase gap in which no stimulation is applied, and then a constant current charge-balancing anodic phase.
Various kinds of neural stimulation performance improvement have been sought by varying the waveform of the signal. For instance variations from the basic symmetric rectangular biphasic current pulse have been investigated for their effect on threshold, for selective recruitment of different sized fibres and for increasing charge delivery capacity of electrodes.
Neural stimulation devices further face certain constraints upon the voltage which may appear on the stimulating electrodes. While miniaturization of neural stimulation devices is desirable, integrated circuit (IC) technologies with reduced device (transistor) size tolerate smaller voltages. Thus, devices using progressively smaller feature semiconductor technologies face limits on the maximum supply voltage available to effect the electrical neural stimulation. While smaller feature semiconductor technologies may be adapted to provide larger electrode voltages, this necessitates increased circuit complexity and power consumption to step up the voltage. Another approach is to use semiconductor fabrication technology which allows both high and low voltage transistors on the same die, however, such specialised devices add to the IC fabrication cost.
Another factor affecting electrode voltage is the need to avoid voltages rising above a certain threshold, to prevent the formation of undesirable chemical products in the tissue surrounding the electrode. One approach for limiting peak electrode voltage involves monitoring the electrode voltage and reducing the current when a prescribed voltage limit is approached.
Implanted devices are generally battery powered and thus have a tight power budget. To reduce the power dissipated in neural stimulation circuitry, careful attention has been paid to design of current sources, and a voltage drive waveform designed to approximately match the electrode voltage under constant current drive has also been proposed. Nevertheless the power budget remains a significant factor in neural stimulation.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.